Toxicity studies were performed with a chemically defined mixture of 25 groundwater contaminants, using dose levels considered to have environmental relevance. The mixture contained 19 organic compounds and six metals. The selection of these compounds was based primarily on the frequency of their occurrence in United States Environmental Protection Agency surveys of groundwater contamination in the vicinity of hazardous waste disposal sites. The report focuses primarily on 26-week drinking water toxicity studies with male and female F344/N rats and B6C3F1 mice. The endpoints evaluated included histopathology, clinical pathology, neurobehavior, and reproductive toxicity. Additional studies using this same chemical mixture are briefly reviewed in the report.